Way Outta Control
Join Date: Sep 1999
Location: The Palouse
Thanked 3 Times in 3 Posts
Re: Voltage guage question
I'd guess your alternator and regulator are OK, or you'd be replacing an overcharged batttery. So let's go over this again in a little more detail talking about current draw and voltage regulation.... with the starter, the headlights. and the heater blower motor. They often cause the voltage meter to report all over the place. Also you've probably seen the headlights dim when the blower motor is turned on untill you bump up the RPM a bit.
The problem you are seeing of the voltage level at the meter on the dash is not unusual. There are voltage drops between the varied subsystems of the Jeep due to ground problems an corrosion on the voltage gauge terminals.
There are three grounds on the Jeep, one cable from the battery to the engine block, a ground strap from the engine block to the frame and a ground strap from the frame to the tub. Also, on some models there is an additional wire connecting the tub firewall to the negative side of the battery. This small wire also acts as a ground.
If any of these are compromised with corrosion, resistance sets in and causes differing problems with the electrical system. As an example on my 1983 CJ7, the negative battery cable was corroded where it affixed to the engine block. All the starting current for the starter motor then went:
From the battery.
Through the cable to the starter solenoid.
Through the starter solenoid to the starter.
Through the starter to the engine block.
Then instead of returning to the battery through the engine block and though the negative battery cable the electricity was forced to return:
Though the frame.
To the engine ground strap.
Through the engine ground strap.
Through the ground strap to the Tub... but that did not exist.
So all the current went though that little tiny wire on the fire wall to the battery.
Well, as you can only imagine, the wire smoked and melted the insulation. It could not handle the current. That's the day I learned about the ground loops in a Jeep.
Of course the other problem endemic to a Jeep is water pouring through the windsshield frame cowl gasket and into the dash area. Then everything behind the dash becomes quite soggy and moist. As it dries out corrosion forms on the connections and voltage drops occur.
Another example would be the heater blower motor fan. As you turn on the fan the blower motor's electric return path is:
From the battery to the fuse box.
Through the ignition switch
Through the supply fuse.
Through the on/off switch.
Through the speed selector.
Through the speed dropping resistors, if selected.
Back through the wring harness and fuse block.
Through the blower motor.
Through the metal back plane of the heater box.
Through the mounting stud of the heater box to the tub firewall.
Through the firewall (tub ground) back to one of the three grounds to the battery.
As you can see, there are several areas that resistance can build up. Just look a the heater box mounting studs. They have star washers on the nuts to help insure a good ground... but over the years corrosion sets in where the star washer has scared the paint. A voltage drop occurs. Now due to a higher resistance of the voltage path, the blower motor pulls more current and the lights dim when it's switched on.
Add to this the lights. Their current path is as follows.
From the battery
Through the fues box.
Through the fuse.
Through the headlight relay
Through the wiring harness
Through the lamp, itself.
Through the return wire to ground.
Through the ground wire to the grill
Remember the grill sits on a bushing mounted to the frame.. usually it's pretty corroded and not a good ground via the bolt the frame.
Through the grill to the fenders.
Through the fenders to the tub ground.
Then back to the battery via one of the three grounds.
Put the headlights on high, and with ANY type of ground loop resistance, a high current application has just become higher. The voltage gauge responds accordingly, as it sees the voltage drop in the tub. The alternator can't keep up with the draw and you see an increase in brightness of the headlights, and an increase in voltage on the gauge, as you increase the RPM's.
In both cases above, however, if you put a volt meter on the battery terminal... I would doubt you'd see a great difference in voltage.
Another source of failure is the thick red wire going from the alternator to the battery side of the starter solenoid. This wire breaks down over time, as it carries all the charging current to the battery and supplies the current to run the vehicle. As it breaks down resistance builds up and it can and will fail.
To make a long note short: First clean ALL the grounds, battery cable connections, alternator connections and voltage gauge terminals. This should restore the accuracy of the ground loops.
Now again put on the lights, do they dim at idle? Probably not.
Now put on the Fan, does it dim the lights at idle? Probably not.
However, if your Voltage gauge is reacting to the current load and the connections have been checked, I suggest changing the meter. There should be a load differential when you first turn on a high current device. It will take a second or two for the alternator to sense the load, and increase the voltage output. That's normal to see the gauge react to the spike. However it's not normal for the gauge to bounce all over it's range, say from 9 volts to 15 volts. This would mean a problem...
But to compound the problem your meter is sticking... even with the engine off it cannot report a steady voltage unless you tap on it. This means the meter movement in the gauge is sticking. As you tap on it, the meter movement is able to overcome the resistance that's causing it to stick and report the higher... or lower voltage, depending upon the swing it was in when it stuck. So, the meter is compromised.
Which is more accurate, a mechanical clock that's broken and doesen't move or an clock that looses or gains time?
Well, the answer is the cloc that's broken is more accurate. It is right twice a day, when the time on the face and the actual time are the same.
The innacurate clock is always close, but never on time, never accurate.
It's much the same for your meter, it's never accurate, and IMHO it never will be. Get rid of the meter, or put an auxilary meter that is accurate, and you can trust, somewhere under the dash.
Happy Holidays and Safe Jeepin'